The invention relates to a radiation detection ionization chamber for measuring radiation in a computer assisted tomographic system (hereinafter sometimes referred to as a CT scanner). The invention also relates to a radiation exposure measuring apparatus for specifically measuring the radiation produced in a CT scanner, and a method for measuring the radiation exposure in a CT scanner.
Computer assisted tomographic scanners, or CT scanners, generally comprise an X-ray, or other radiation, source, and a detector positioned at opposite sides of a space in which a part of a human body is positioned. The source and detector are adapted to rotate about the body to scan the body. An x-ray slice can be obtained by such a scan and the information directed to a computer wherein the signals are processed to form a reproduction of the internal portion of the human body being scanned. Such CT scanners can comprise a single radiation source and a single detector, or a plurality of sources and detectors, wherein the later arrangement produces a plurality of X-ray slices with a single scan. A description of CT scanning technique and theory is provided in "Introduction to Computer Tomography" copyrighted 1976, published by General Electric Co.
With the advent of the CT scanner, it has become important to provide for some means to measure the radiation produced in a scanner to determine the exposure levels, or dosage, on the object to be scanned. To this date, there has been no uniform apparatus or method used to measure the X-ray radiation dosage produced by the CT scanner, and it is an object of this invention to provide for an apparatus and method for measuring the exposure levels in a CT scanner.
One current method employed for measuring exposure, or dose, and the dose distributions due to CT scanning, is to use a large number of thermo-luminescent detectors (TLDs) that are placed around a phantom. However, such TLDs have disadvantages over an ionization chamber with regard to precision, accuracy, and ease of reading.
In order to properly measure the exposure of a CT scanner, it is an object of this invention to provide a probe that is able to detect radiation directed from all angles of 360.degree.. It is further an object of this invention to obtain an exposure reading using only a single scan of the CT scanner. Still further, it is an object of the invention to provide a method and apparatus that provides for the dosage intercomparison of different CT scanners.
The detection chamber of the present invention is an elongated pencil shaped chamber comprising an elongated cylindrical plastic tubing having a length along a major axis of the tubing substantially greater than the diameter of the tubing. Positioned centrally along the major axis of the tubing is an elongated center electrode, formed of air equivalent plastic. At one of the ends of the tubing is a cable connector structure which is designed to connect the center electrode to a conductor of an electrical cable and to connect the tubing to another conductor of the electrical cable. At the opposite end of the chamber is a support structure for supporting the center electrode within the tubing and to provide tensioning of the center electrode between the cable connector and the support structure.
It is further an object of this invention to provide a new and unique radiation detection chamber for use in a CT scanner. Moreover, it is an object of this invention to provide the radiation detection chamber in a pencil-sized shape that is compact, inexpensive and easy to assemble and calibrate.
It is still further an object of this invention to provide a detection chamber having a center electrode of air equivalent plastic to be positioned in a tensioned state and capable of being exposed to radiation in a full 360.degree. circle.
It is yet another object of this invention to provide a new and unique structure for clamping and tensioning the center electrode of a detection chamber and for connecting the center electrode to an electrical cable, such as a triaxial cable, in order that the device can be simply and quickly positioned within the CT scanner and interconnected, by means of the electrical cable, to suitable and conventional signal processing circuits to detect the current induced in the center electrode as a result of the radiation impinging upon the chamber.
Another embodiment of the invention relates to an apparatus for measuring the radiation produced in a CT scanner. The apparatus comprises a phantom block designed to approximate the size of the human body portion that is to be scanned in order to determine the radiation impinging on the human body portion, and to provide suitable means for attaching the detection chamber to the phantom. The phantom comprises a block of preferably polystyrene material that is shaped so as to be positioned within the patient positioning space of the CT scanner. The phantom can be a single unitary block or can be composed of a plurality of block portions fixedly secured to each other to form a unitary structure. The chamber can be attached to the block at the exterior around the periphery of the block, or the block can be provided with holes in which the elongated pencil chamber is positioned.
The invention also relates to a method for measuring radiation in a CT scanner by using the apparatus discussed above. By attaching an elongated pencil detection chamber to a phantom block approximating the size of the object to be scanned, and scanning the phantom and chamber, one can obtain readings to measure the radiation dose of the scanner.
The pencil-type chamber has applications other than for a CT scanner. The chamber can be used as a linear scale (ruler) for field size measurements to determine 50% exposure boundaries in a diagnostic or thereapeutic system. This is a very useful tool for a field service man who has to align the light field indicator to the X-ray field.